If a gust of wind came and carried my hat into your meadow would you keep it?

I would have expected "dá dtiocfadh..." or some other construction in the conditional.

Was this usage more common once? Does it convey a different shade of meaning to the conditional?

Go raibh maith agaibh

It isn't imperfect tense but past subjunctive (foshuiteach caite). Both tenses have the same form (imperative has the same form as well in 3rd person) except for the verb bí (here conditional and past subjunctive have the same form: bheadh, whereas bíodh is used in imperfect/imperative)

The conjunctions dá and mura were followed traditionally by the past subjunctive. Only the consequence of the condition was in the conditional. Dá dtigeadh/dtagadh sí, thiocfadh sé freisin. = If she would come, he would come, too